Unpaid losses and loss adjustment expenses - Incurred and paid losses and allocated loss adjustment expenses (Detail) Claim in Thousands, $ in Millions		Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	[1]	Dec. 31, 2014 USD ($)	[1]	Dec. 31, 2013 USD ($)	[1]	Dec. 31, 2012 USD ($)	[1]	Dec. 31, 2011 USD ($)	[1]	Dec. 31, 2010 USD ($)	[1]	Dec. 31, 2009 USD ($)	[1]	Dec. 31, 2008 USD ($)	[1]	Dec. 31, 2007 USD ($)	[1]
Claims Development [Line Items]
Net unpaid losses and ALAE		$ 47,299
Insurance Group [Member] | GEICO [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		74,994
Cumulative Paid Losses and ALAE		62,555
Net unpaid losses and ALAE		12,981
Insurance Group [Member] | GEICO [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		11,900	$ 11,906		$ 11,893		$ 11,904		$ 12,034
IBNR and Case Development Liabilities		$ 79
Cumulative Number of Reported Claims | Claim		6,459
Cumulative Paid Losses and ALAE		$ 11,609	11,311		10,744		9,832		7,550
Insurance Group [Member] | GEICO [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		12,837	12,859		12,815		12,990
IBNR and Case Development Liabilities		$ 184
Cumulative Number of Reported Claims | Claim		7,102
Cumulative Paid Losses and ALAE		$ 12,174	11,569		10,494		7,944
Insurance Group [Member] | GEICO [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		14,477	14,488		14,597
IBNR and Case Development Liabilities		$ 539
Cumulative Number of Reported Claims | Claim		7,965
Cumulative Paid Losses and ALAE		$ 13,060	11,956		9,133
Insurance Group [Member] | GEICO [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		16,798	16,807
IBNR and Case Development Liabilities		$ 1,394
Cumulative Number of Reported Claims | Claim		8,891
Cumulative Paid Losses and ALAE		$ 13,785	10,543
Insurance Group [Member] | GEICO [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		18,982
IBNR and Case Development Liabilities		$ 3,132
Cumulative Number of Reported Claims | Claim		9,336
Cumulative Paid Losses and ALAE		$ 11,927
Insurance Group [Member] | GEICO [Member] | 2012-2016 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE		12,439
Insurance Group [Member] | GEICO [Member] | Before 2012 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	[1]	542
Insurance Group [Member] | General Re [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		16,818
Cumulative Paid Losses and ALAE		10,407
Net unpaid losses and ALAE		13,973
Insurance Group [Member] | General Re [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,535	1,603		1,707		1,850		1,810
IBNR and Case Development Liabilities		335
Cumulative Paid Losses and ALAE		991	916		829		664		199
Insurance Group [Member] | General Re [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,854	1,994		2,095		1,945
IBNR and Case Development Liabilities		417
Cumulative Paid Losses and ALAE		1,162	1,060		829		275
Insurance Group [Member] | General Re [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,794	1,843		1,740
IBNR and Case Development Liabilities		578
Cumulative Paid Losses and ALAE		864	666		171
Insurance Group [Member] | General Re [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,999	1,811
IBNR and Case Development Liabilities		777
Cumulative Paid Losses and ALAE		693	207
Insurance Group [Member] | General Re [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,715
IBNR and Case Development Liabilities		1,173
Cumulative Paid Losses and ALAE		171
Insurance Group [Member] | General Re [Member] | 2007 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,512	1,535		1,559		1,574		1,619		$ 1,673		$ 1,746		$ 1,852		$ 1,915		$ 1,857
IBNR and Case Development Liabilities		73
Cumulative Paid Losses and ALAE		1,335	1,322		1,309		1,291		1,238		1,195		1,132		1,020		821		282
Insurance Group [Member] | General Re [Member] | 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,635	1,648		1,669		1,713		1,746		1,818		1,903		2,011		1,920
IBNR and Case Development Liabilities		93
Cumulative Paid Losses and ALAE		1,354	1,330		1,315		1,289		1,246		1,187		1,074		894		330
Insurance Group [Member] | General Re [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,431	1,471		1,490		1,523		1,601		1,704		1,790		1,703
IBNR and Case Development Liabilities		125
Cumulative Paid Losses and ALAE		1,161	1,145		1,115		1,065		994		897		734		264
Insurance Group [Member] | General Re [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,674	1,712		1,757		1,845		1,939		2,055		1,881
IBNR and Case Development Liabilities		156
Cumulative Paid Losses and ALAE		1,359	1,304		1,243		1,147		997		782		247
Insurance Group [Member] | General Re [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,669	1,744		1,792		1,897		2,126		2,102
IBNR and Case Development Liabilities		208
Cumulative Paid Losses and ALAE		1,317	1,259		1,190		1,086		841		302
Insurance Group [Member] | General Re [Member] | 2007-2016 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE		6,411
Insurance Group [Member] | General Re [Member] | Before 2007 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	[1]	7,562
Insurance Group [Member] | BHRG [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	28,948
Cumulative Paid Losses and ALAE	[2]	19,333
Net unpaid losses and ALAE	[2]	10,172
Insurance Group [Member] | BHRG [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	3,531	3,583		3,648		3,878		4,054
IBNR and Case Development Liabilities		694
Cumulative Paid Losses and ALAE	[2]	2,519	2,350		2,080		1,279		360
Insurance Group [Member] | BHRG [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	2,832	2,935		3,144		3,326
IBNR and Case Development Liabilities		733
Cumulative Paid Losses and ALAE	[2]	1,774	1,555		1,070		516
Insurance Group [Member] | BHRG [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	2,492	2,585		2,688
IBNR and Case Development Liabilities		736
Cumulative Paid Losses and ALAE	[2]	1,319	1,031		437
Insurance Group [Member] | BHRG [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	3,070	3,207
IBNR and Case Development Liabilities		1,113
Cumulative Paid Losses and ALAE	[2]	1,354	550
Insurance Group [Member] | BHRG [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	3,390
IBNR and Case Development Liabilities		1,907
Cumulative Paid Losses and ALAE	[2]	778
Insurance Group [Member] | BHRG [Member] | 2007 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	1,471	1,500		1,560		1,583		1,590		1,652		1,658		1,659		1,756		1,810
IBNR and Case Development Liabilities		133
Cumulative Paid Losses and ALAE	[2]	1,221	1,211		1,196		1,170		1,120		1,055		993		891		677		171
Insurance Group [Member] | BHRG [Member] | 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	2,578	2,605		2,683		2,748		2,835		2,903		2,988		3,130		3,388
IBNR and Case Development Liabilities		244
Cumulative Paid Losses and ALAE	[2]	2,228	2,205		2,131		2,056		1,976		1,836		1,617		1,146		364
Insurance Group [Member] | BHRG [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	2,668	2,691		2,759		2,847		2,926		2,989		2,858		2,974
IBNR and Case Development Liabilities		215
Cumulative Paid Losses and ALAE	[2]	2,363	2,336		2,306		2,147		1,937		1,605		1,182		372
Insurance Group [Member] | BHRG [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	2,551	2,575		2,610		2,771		2,883		2,974		2,876
IBNR and Case Development Liabilities		238
Cumulative Paid Losses and ALAE	[2]	2,177	2,119		2,041		1,736		1,414		889		193
Insurance Group [Member] | BHRG [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	[2]	4,365	4,389		4,440		4,358		4,544		4,418
IBNR and Case Development Liabilities		452
Cumulative Paid Losses and ALAE	[2]	3,600	3,475		3,315		2,872		2,130		552
Insurance Group [Member] | BHRG [Member] | 2007-2016 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE	[2]	9,615
Insurance Group [Member] | BHRG [Member] | Before 2007 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	[1],[2]	557
Insurance Group [Member] | BH Primary [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		19,184
Cumulative Paid Losses and ALAE		9,408
Net unpaid losses and ALAE		10,173
Insurance Group [Member] | BH Primary [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,903	1,944		2,017		2,079		2,129
IBNR and Case Development Liabilities		$ 453
Cumulative Number of Reported Claims | Claim		135
Cumulative Paid Losses and ALAE		$ 1,208	1,053		844		598		211
Insurance Group [Member] | BH Primary [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		2,060	2,133		2,213		2,294
IBNR and Case Development Liabilities		$ 595
Cumulative Number of Reported Claims | Claim		150
Cumulative Paid Losses and ALAE		$ 1,197	985		706		350
Insurance Group [Member] | BH Primary [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		2,730	2,780		2,967
IBNR and Case Development Liabilities		$ 1,027
Cumulative Number of Reported Claims | Claim		177
Cumulative Paid Losses and ALAE		$ 1,247	896		453
Insurance Group [Member] | BH Primary [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		3,458	3,575
IBNR and Case Development Liabilities		$ 1,631
Cumulative Number of Reported Claims | Claim		188
Cumulative Paid Losses and ALAE		$ 1,078	502
Insurance Group [Member] | BH Primary [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		4,149
IBNR and Case Development Liabilities		$ 2,682
Cumulative Number of Reported Claims | Claim		135
Cumulative Paid Losses and ALAE		$ 634
Insurance Group [Member] | BH Primary [Member] | 2007 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		775	788		817		852		901		955		1,090		1,174		1,233		1,328
IBNR and Case Development Liabilities		$ 46
Cumulative Number of Reported Claims | Claim		137
Cumulative Paid Losses and ALAE		$ 705	696		679		660		628		578		510		409		284		$ 128
Insurance Group [Member] | BH Primary [Member] | 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		920	948		979		1,015		1,081		1,168		1,228		1,273		1,349
IBNR and Case Development Liabilities		$ 80
Cumulative Number of Reported Claims | Claim		136
Cumulative Paid Losses and ALAE		$ 810	795		772		728		662		578		464		333		$ 174
Insurance Group [Member] | BH Primary [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		936	966		1,020		1,068		1,159		1,180		1,211		1,298
IBNR and Case Development Liabilities		$ 99
Cumulative Number of Reported Claims | Claim		117
Cumulative Paid Losses and ALAE		$ 798	771		726		655		569		445		305		$ 147
Insurance Group [Member] | BH Primary [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		956	994		1,062		1,129		1,191		1,186		1,268
IBNR and Case Development Liabilities		$ 150
Cumulative Number of Reported Claims | Claim		111
Cumulative Paid Losses and ALAE		$ 758	722		657		570		458		313		$ 146
Insurance Group [Member] | BH Primary [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE		1,297	1,375		1,452		1,605		1,633		1,888
IBNR and Case Development Liabilities		$ 223
Cumulative Number of Reported Claims | Claim		109
Cumulative Paid Losses and ALAE		$ 973	$ 898		$ 753		$ 602		$ 396		$ 163
Insurance Group [Member] | BH Primary [Member] | 2007-2016 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE		9,776
Insurance Group [Member] | BH Primary [Member] | Before 2007 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	[1]	$ 397

[1] Unaudited supplemental information

[2] Excludes retroactive reinsurance losses and ALAE